High-power laser diodes require relatively high current at relatively low voltages. Laser diodes tend to have not only the non-linear impedance characteristics of a semiconductor diode, but also require a large threshold current to initiate and sustain lasing. However, laser diodes are sensitive to overcurrent conditions. In pulsed laser-diode operation, a power supply having a large capacitance at its output has a possibility of delivering undesirable excessive current when initially connected to a laser diode, resulting in catastrophic damage to the laser diode. If a series-pass regulator is used to limit current or voltage, a large voltage drop can develop across the regulator, resulting in inefficient power usage.
Some traditional methods of making constant-current drivers provide a variable-voltage supply that can be adjusted using a control circuit to hold the output current constant. This method works reasonably well for constant-impedance DC loads (such as resistors) and for applications that do not require fast rise time. Typical power-supply circuits of this type provide a voltage supply and typically have significant output capacitance. The load cannot be switched on with a fast rise time since the power-supply circuit must charge the output capacitance. If the load (such as a laser diode) is ever momentarily disconnected during operation, the control circuit, which continues to supply current, will overcharge the output capacitor. When the load then reconnects, the inevitable current surge available from the discharge of the output capacitor, and possibly additional current from the control circuit, will most likely destroy a laser diode.
Another conventional current-source circuit topology is a voltage supply with a series-pass linear current regulator. Typical power-supply circuits of this type can provide fast rise times and can have little or no significant output capacitance. One penalty is that the series-pass regulator will typically require a 2- to 6-volt drop (or more) across the regulator itself to act as an effective regulator. Power-transfer efficiencies of this circuit thus can be very poor.
What is needed is a high-efficiency power supply capable of selectively delivering large-well-regulated currents at low voltages. Also needed, in some embodiments, is the ability to specify a desired current level, the ability to change the current level, and/or the ability to turn the current on and off quickly.